1. Field of the Invention
The present invention relates in general to piezoelectric flexure mode devices, and more particularly to such a device (called a "unimorph") comprising a layer of piezoelectrically active material bonded to a layer of piezoelectrically inactive material.
2. Discussion Related to the Problem
A piezoelectric flexure mode device comprises a plurality of layers of material of differing piezoelectric activity. Under the application of an electric field across the thickness of such a device, small differences in the amounts of expansion or contraction in the planes of the different layers are converted into large deflections of the layers out of their planes. Alternatively, if an external force is applied to the device, causing it to flex, a voltage is generated across the layers of the device. Piezoelectric flexure mode devices have found utility as electrical-to-mechanical and mechanical-to-electrical transducers in such diverse applications as speakers, microphones, phonograph cartridges, motors, and accelerometers.
In its simplest form, a piezoelectric flexure mode device comprises a structure of two layers bonded together, one of the layers exhibiting piezoelectric activity, and the other not. In the presence of an electric field (generally applied by means of electrodes fixed to the top and bottom surfaces of the device), the piezoelectric layer expands or contracts in its own plane, and because the other layer, which is not piezoelectrically active, resists the expansion or contraction at the bonded interface between the two layers, the device flexes or bends. This form of flexure mode device is known as a "unimorph", and in a common embodiment comprises a slab of piezoelectric ceramic material, such as lead zirconate titanate (PZT), cemented to a sheet of metal such as brass or copper. The metal sheet serves as the piezoelectrically inactive layer, and as one of the electrodes of the device.
A more common form of flexure mode device, known as a bimorph, comprises two layers of piezoelectric material arranged so that when one layer expands, the other contracts, thereby producing more deflection for a given applied field. Since there are two piezoelectrically active layers of material in the bimorph, as opposed to one in the unimorph, the bimorph may be expected to perform twice as well as the unimorph.
For further information on the structural details of prior art flexure mode piezoelectric devices, see the Article by C. P. Germano entitled "Flexure Mode Piezeoelectric Transducers", IEEE Transactions on Audio and Electroacoustics, Vol. Au-19, No. 1, March, 1971.
In some applications proposed for piezoelectric flexure mode devices, e.g., laser scanners and visual displays, relatively large deflections and fast response times are required of the flexure mode devices. A figure of merit, useful for comparing the suitability of different flexure mode devices in such applications requiring both large deflections and high speed, is defined as the DC (i.e. very low frequency) deflection of the device times the first resonant frequency of the device and is called the Deflection Bandwidth Product (DBWP).
Recently, piezoelectric polymeric materials such as polyvinylidene fluoride (PVF.sub.2) have received considerable attention for use in various applications. Although the piezoelectric constant d.sub.31 (a constant that specifies the amount of strain in the plane of a sheet of piezoelectric material produced by an electric field perpendicular to the plane) in this material is considerably lower than that of a ceramic piezoelectric material such as PZT, the DBWP for a flexure mode device made with PVF.sub.2 is higher than the DBWP for such a device made with PZT. Hence, flexure mode devices made with polymeric piezoelectric materials are prime candidates for applications requiring large deflections and fast response. Another factor making polymeric piezoelectric materials very attractive, is their plastic properties, since well developed plastics manufacturing technology can be readily adapted to the economical manufacture of polymeric piezoelectric devices.
In making a polymeric piezoelectric flexure mode device, the "unimorph" construction is attractive because of its structural simplicity. On the other hand, the bimorph construction is desirable because of its better performance. One of the problems with either construction, however, is bonding the layers of piezoelectric polymeric material to each other, or to a metal sheet. Problems have been encountered both in achieving sufficient adhesion of the bonding material to the polymeric material and the metal, and in matching the mechanical impedence of the bonding material with that of the polymeric piezoelectric material.
An improper impedence match, or a faulty bond impairs the efficiency of the flexure mode device. The problem faced by the inventors therefore was to provide an efficient, simple, flexure mode device, capable of producing relatively large deflections and having a relatively fast response time, using polymeric piezoelectric material.